Typically, a computer system includes a number of integrated circuit chips that communicate with one another to perform system applications. Often, the computer system includes a controller, such as a micro-processor, and one or more memory chips, such as random access memory (RAM) chips. The RAM chips can be any suitable type of RAM, such as dynamic RAM (DRAM), double data rate synchronous DRAM (DDR-SDRAM), low power SDRAM (LP-SDRAM), and/or pseudo static RAM (PSRAM). The controller and memory communicate with one another to perform system applications.
Some computer systems operate in mobile applications and have limited space and power resources. In mobile applications, such as cellular telephones and personal digital assistants (PDAs), memory cell density and power consumption are issues for current and future generations. Low power mobile RAM is a LP-SDRAM that has been developed for mobile applications. CellularRAM is a high performance and low power memory designed to meet memory density and bandwidth demands of current and future generations. CellularRAM is a PSRAM that offers static RAM (SRAM) pin and function compatibility, external refresh-free operation, and a low power design. CellularRAM devices are drop-in replacements for most asynchronous low power SRAMs used in mobile applications, such as cellular telephones. Usually, a PSRAM is based on a DRAM that provides significant advantages in density and speed over traditional SRAM.
Typically, each DRAM memory cell includes one metal oxide semiconductor (MOS) transistor and one capacitor that stores one bit of data via an electrical charge. However, this electrical charge leaks from the capacitor over time such that the capacitor must be refreshed periodically to maintain data integrity. Self refresh operations are automatically executed at certain intervals to maintain data integrity where the DRAM memory cell has not been accessed for a long period.
Partial array self refresh (PASR) is an operation where self refresh operations are not performed on all memory cells in a memory, but on only some memory cells or memory banks where data retention is required. For example, in one PASR setting only one memory bank may be refreshed in a CellularRAM having four memory banks. Data stored outside the refresh area is not retained. PASR reduces self refresh current to achieve lower power consumption. PASR settings can be switched without powering down the DRAM. However, switching PASR settings can lead to loss of data.
For these and other reasons there is a need for the present invention.